Exploring the potential of standalone and tandem solar cells with Sb2S3 and Sb2Se3 absorbers: a simulation study

Thin-film antimony chalcogenide binary compounds are potential candidates for efficient and low-cost photovoltaic absorbers. This study investigates the performance of Sb2S3 and Sb2Se3 as photovoltaic absorbers, aiming to optimize their efficiency. The standalone Sb2S3 and Sb2Se3 sub-cells are analyzed using SCAPS-1D simulations, and then a tandem structure with Sb2S3 as the top-cell absorber and Sb2Se3 as the bottom-cell absorber is designed, using the filtered spectrum and the current matching technique. The optimal configuration for maximum efficiency is achieved by adjusting the thickness of the absorber layer. The results show that antimony chalcogenide binary compounds have great potential as photovoltaic absorbers, enabling the development of efficient and low-cost solar cells. A remarkable conversion efficiency of 22.2% is achieved for the optimized tandem cell structure, with absorber thicknesses of 420 nm and 1020 nm for the top and bottom sub-cells respectively. This study presents a promising approach towards high-performance tandem solar cells.


Methodology
An example of a single junction solar cell is shown in Fig. 1.As the absorber layer, this device contains Sb 2 S 3 or Sb 2 Se 3 , CdS as the electron transport layer (ETL), and SpiroOMeTAD as the hole transport layer (HTL), as well as Au and FTO acting as back and front contacts, respectively.Simulations of single junction devices were conducted using the SCAPS software.The program is based on numerical solutions of the Poisson equation, current density equations for electrons and holes, and continuity equations for holes and electrons with appropriate boundary conditions at interfaces and contacts 13,35 .
In the first step, a model based on experimental data 22 was used to design independent single junction subcells comprised of SnO 2 :F/ETL (CdS)/absorber/HTL (SpiroOMeTAD)/Au.Based on the previous research work reported in the literature, we have selected the material parameters and presented them in Table 1S 13,33,36 .We have used different dielectric permittivity and electron and hole mobility for the Sb 2 S 3 /Sb 2 Se 3 absorber layer 25 .A neutral defect with a density of 1.4 × 10 12 cm −2 was assumed at both the ETL/absorber and HTL/absorber interfaces.We have also taken into account the bulk defects reported for record Sb 2 (S, Se) 3 solar cells in the absorber layers 22 .In addition, we have assumed an internal defect near the midgap states for HTL, ETL, and FTO layers.The defect state parameters for the absorber, HTL, ETL, and FTO layers are shown in Table 2S.We have obtained all the other input parameters for the simulation of solar cells from the literature 36 or logically assumed them to prevent misleading or unrealistic results.The spectral conditions for all simulations are 1.5 AM and the operating temperature is 300 K. Table 3S shows a comparison of the experimental and simulated solar cells data.The good consistency between the simulated results and the experimental data confirms the validity of our model.
There are two types of absorber layers in the top and bottom sub-cells, with Sb 2 S 3 on the top and Sb 2 Se 3 on the bottom.A relationship was established between the performance of the sub-cells and the thickness of the absorber layer.We used the AM 1.5 G spectrum and a temperature of 300 K for standalone simulations of top and bottom sub-cells.
An absorber layer's band gap determines the number of photons that can be absorbed by a single junction solar cell.It is possible to enhance the performance of photovoltaic systems by using tandem structures.In tandem cells containing Sb 2 S 3 /Sb 2 Se 3 , the top cell contains an absorber layer with a band gap of 1.7 eV and the bottom cell contains an absorber layer with a band gap of 1.2 eV.After evaluating the performance of Sb 2 S 3 -based top and Sb 2 Se 3 -based bottom cells, the Sb 2 S 3 /Sb 2 Se 3 tandem structure was analyzed.
We can simulate a tandem architecture in SCAPS by connecting the top and bottom cells using the current matching technique 23 .The Sb 2 S 3 top cell is illuminated by the standard global AM 1.5 G spectrum; however, the Sb 2 Se 3 bottom cell is illuminated by the residual solar spectrum after partial absorption across the top cell.Using this equation 24 , we can calculate the spectrum passing through the top sub-cell with varying Sb 2 S 3 thicknesses to the bottom sub-cell: This is the incident standard global AM 1.5 G spectrum, the layer number, represents the number of layers of the top sub-cell, and the thickness of each layer represents T0(λ), k, n and t respectively.Also, α(λ) is the absorption coefficient of each material and given by 24 : where Aα, Eg, h, and ν are pre-factor, the energy gap of the material (eV), the Planck's constant (eV.sec), and the spectrum frequency respectively.The Fig. 2 illustrates the schematic of a Sb 2 S 3 /Sb 2 Se 3 tandem device with Each sub-cell has an absorber layer that absorbs photons with an energy greater than the band gap.The thickness of top and bottom sub-cells was altered in order to achieve equal current density across both.The monolithic tandem solar cell must meet this requirement.

Simulation of Sb 2 S 3 top cell
This section focuses on simulating the Sb 2 S 3 top sub-cell.Simulations of top sub-cells were conducted using the standard AM 1.5 G spectrum.A study was conducted to examine the effect of the thickness of the absorber layer on the Sb 2 S 3 sub-cell.The thickness of the absorber layer was varied between 50 nm and 2 µm.Figures 3 and 4 present the J-V characteristics, external quantum efficiency (EQE), and photovoltaic parameters, respectively, of Sb 2 S 3 solar cells with varying absorber layer thicknesses.By increasing the absorbance layer thickness to 650 nm, J SC increased to 21.2 mA cm −2 as a result of increased absorption and generation rates.Further increases in thickness beyond 650 nm, however, resulted in saturation of absorption and little change in J SC values.There is an increase in recombination in the thicker Sb 2 S 3 absorber as a result of the addition of a 1.25 µm thick Sb 2 S 3 layer 23 .According to external quantum efficiency spectra, photon harvesting was improved up to 1-1.2 µm, after which it began to decline for wavelengths less than 730 nm.As a result of this decrease in response, J SC values decreased.As the thickness of the Sb 2 S 3 absorber increased from 50 nm to 2 µm, V OC reduced slightly from 590 to 545 mV and fill factor (FF) from 70.42 to 28.47%.The reduced electric field across the thick Sb 2 S 3 layer is the cause of the decreased V OC and FF in the device (Fig. 5).The lower electric field reduces the probability of charge carrier separation, thus reducing the V OC and FF 23,37 .An increase in J SC increases power conversion efficiency (PCE) for thicknesses less than 350 nm.In contrast, when it comes to thicker films, it is the reduction of V OC s and FF that becomes the determining factor.Accordingly, PCE was initially increased, followed by a decrease as thickness increased, with a maximum PCE value of 6.65% at 350 nm thick Sb 2 S 3 .

Simulation of Sb 2 Se 3 bottom cell
In this section, the study of simulating the Sb 2 Se 3 bottom sub-cell is performed.For standalone simulations of bottom sub-cell, the standard AM 1.5 G spectrum was employed.J-V characteristics of the Sb  6a.Extracted from the different J-V characteristics in Fig. 6, the photovoltaic parameters of Sb 2 S 3 solar cells with different absorber thickness is illustrated in Fig. 7.We discovered that Sb 2 Se 3 solar cells have a relatively lower open circuit voltage than Sb 2 S 3 absorber layer-based solar cells, as shown in Fig. 7a, because Sb 2 Se 3 has a smaller band gap than Sb 2 S 3 absorber layer 38,39 .A slight increase in the V OC of Sb 2 Se 3 solar cells is observed as thickness of absorber grows, reaching its maximum value.This is mostly due to the fact that a thicker Sb 2 Se 3 absorber layer will absorb more photons, contributing to the production of electron-hole pairs 40 .Figure 7a shows that increasing the absorbance layer thickness up to 700 nm resulted in an increase in J SC to 38.0 mA cm −2 due to enhanced absorption and generation rates.However, further increases in thickness beyond 700 nm led to saturation of absorption and little change in J SC values.The external quantum efficiency spectra of Sb 2 S 3 sub-ells with various thicknesses are depicted in Fig. 6b.The absorber layer's photon harvesting improved with an increase in thickness of up to 0.8 µm.However, as thickness rises above this point, the optical absorption virtually reaches saturation.This also verifies the variation in the J SC as mentioned earlier.In Fig. 7b, it is visible that the FF is greatly dropped from 63.4% for 50 nm to 48.9% for 2.0 µm.As the thickness of these single junction solar cells increases, so does their efficiency, peaking at 0.5-0.7 µm.After that, efficiency starts to drop off as thickness continues to increase.‫‬

Simulation of Sb 2 S 3 /Sb 2 Se 3 tandem solar cell
In this part, the performance of Sb 2 S 3 /Sb 2 Se 3 tandem solar cells is explored utilizing a filtered spectrum (described in the "Methodology" section) combined with a current matching process.The filtered spectrum and integrated power are shown in Fig. 8 for different thicknesses of the absorber layer in the top cell.The filtered spectrum clearly illustrates that with increasing Sb 2 S 3 thickness, power passed to the bottom cell decreases, especially at wavelengths below 730 nm.Furthermore, Fig. 8b shows that the integrated power transmitted by the top cell decreases as the thickness of the absorber layer decreases.
Having equal currents in both sub-cells is crucial for monolithic tandem solar cells 23,24 .Thus, both the upper and lower sub-cell thicknesses must be adjusted at the same time in order to achieve current matching.For each sub-cell, the thickness of the top cell was altered in 100 nm steps while the thickness of the bottom cell was changed until an identical J SC was obtained.In the top cell, AM 15 G spectrum is used to illuminate the cell, and in the bottom cell, a filtered spectrum is used to illuminate the cell.Therefore, the J SC for the lower sub-cell is altered as a result of the thickness of the absorb layer in both cells, while for the upper cell the J SC is altered as a result of the thickness of the absorb layer in only one cell.
J-V curves are presented in Fig. 9 for different thicknesses of Sb 2 S 3 /Sb 2 Se 3 tandem solar cells.At different current matching points, Fig. 10 illustrates the fluctuation of photovoltaic characteristics of tandem solar cells.Compared to the single device, the tandem device exhibits improved values of J SC at higher current matching points.With a 430 nm (top absorber)/1310 nm (bottom absorber) configuration, the highest J SC value was achieved.Current matching requires thicker bottom absorbers for thicker top absorbers; for example, 450 nm thick top absorbers require 6 µm thick bottom absorbers.In order to minimize material consumption, keeping the top layer within 400-430 nm is more efficient.A comparison of V OC of tandem solar cells for different current matching points shows that V OC to FF ratios decrease at higher current matching points.As shown in Fig. 10b, the PCE of the tandem cell under matching conditions can be determined.As cell thickness increases,  www.nature.com/scientificreports/efficiency increases until it reaches a plateau at or beyond 400 nm for the upper sub-cell.420 nm and 1020 nm are the optimal thicknesses for the top and bottom sub-cells of a tandem device (Fig. 10b).The efficiency of the system becomes saturated when the value is exceeded.Figure 2 illustrates the structure of this device, illuminated by AM 1.5 G incident spectrum on the top sub-cell of Sb 2 S 3 and filtered AM 1.5 G on the bottom sub-cell of Sb 2 Se 3 .Figure 11 shows the J-V curve of each sub-cell along with their combined tandem solar cell counterpart.Figure 12 illustrates the spectrum of Sb 2 S 3 , Sb 2 Se 3 , and the Sb 2 S 3 /Sb 2 Se 3 tandem device.Additionally, Fig. 13 illustrates the external quantum efficiency of both sub-cells.As illustrated by the quantum efficiency curve in Fig. 13, photon energy was more effectively taken in at longer wavelengths from the bottom cell with a lower band gap and shorter wavelengths from the top cell with a higher band gap, resulting in the generation of current at a wide range of frequencies.At wavelengths up to 729 nm (hν > 1.7 eV), the upper cell is shown to be dominant in the EQE versus wavelength plots.For longer wavelengths (hν < 1.7 eV) however, the upper cell becomes virtually invisible and absorption from the bottom cell increases, resulting in improved quantum efficiency across a broad range.

Conclusions
Photovoltaic systems can be improved by using tandem structures.The performance of the Sb 2 S 3 and Sb 2 Se 3 sub-cells when operating independently was examined, after which the tandem structure of Sb 2 S 3 /Sb 2 Se 3 was explored.The tandem structure had a power conversion efficiency that was much higher than the standalone cells.In order to achieve "current matching"-a point at which thesub-cells have the same J SC -and reduce current loss due to amismatch of current density, it was necessary to change the thickness of the absorber in tandem cells.For thicker top absorbers, higher thicknesses of bottom absorbers are required for current matching.To minimize material usage we propose that the thickness of top layer choose within 400-430 nm range.This Sb 2 S 3 /Sb 2 Se 3 tandem solar cell yielded an impressive conversion efficiency of 22.2%, with 420 nm and 1020 nm thick absorber layers for its top and bottom sub-cells respectively.The other photovoltaic parameters are J SC (20.15 mA cm −2 ), V OC (1.02 V), and FF (59.58%).This research on the Sb 2 S 3 /Sb 2 Se 3 tandem design can lead to the production of high-performance tandem solar cells.

Figure 1 .
Figure 1.Device architecture of Sb 2 (S,Se) 3 solar cell used in the numerical simulation.

Figure 2 .
Figure 2. Schematic diagram of tandem cell with (a) AM1.5 spectrum at the top sub-cell and (b) filtered spectrum transmitted by top sub-cell with different absorber layer thickness at bottom sub-cell.

Figure 4 .Figure 5 .
Figure 4. Photovoltaic characteristics of Sb 2 S 3 based top cell with absorber thickness: (a) V OC and J SC and (b): FF and PCE.

Figure 6 .
Figure 6.(a) J-V curve and (b) EQE of Sb 2 Se 3 based bottom cell with absorber thickness in a standalone configuration.

Figure 7 .Figure 8 .
Figure 7. Photovoltaic characteristics of Sb 2 Se 3 based bottom cell with absorber thickness: (a) V OC and J SC and (b): FF and PCE.

FFFigure 10 .
Figure 10.Photovoltaic characteristics of Sb 2 S 3 /Sb 2 Se 3 tandem solar cell with thicknesses corresponding to the first seven current matching points: (a) V OC and J SC and (b): FF and PCE.

Figure 11 .Figure 12 .
Figure 11.Illuminated J-V curves of standalone Sb 2 S 3 top and Sb 2 Se 3 bottom sub-cells at optimized thickness (420 nm/1020 nm), bottom cell under filtered spectrum by top cell, and Sb 2 S 3 /Sb 2 Se 3 tandem solar cell.
J-V curve of Sb 2 S 3 /Sb 2 Se 3 tandem with thicknesses corresponding to first seven current matching points.